1. Field of the Invention
The present invention relates to a star-quad cable for transmitting electrical signals which has at least two pairs of electrical conductors, each conductor having a core made of an electrically conductive material and a conductor sheath made of an electrically insulating material which surrounds the core in a radial position, the conductors being arranged at the corners of a square in a cross-section of the star-quad cable, the conductors making up a pair being arranged at diagonally opposed corners of the square, four conductors at a time being twisted together in a star-quad arrangement with a predetermined lay factor, a shield made of an electrically conductive material which surrounds the two pairs of conductors on the outside radially being placed in position, and an additional insulator sheath made of an electrically insulating material being arranged between the conductors and the shield, the shield being constructed from a mesh of individual shield cores.
2. Description of Related Art
What is referred to as a “star-quad” is a lay-up term relating to conductors which have for example copper cores. Four conductors making up pairs of conductors are twisted together and then form two twin conductors which are laid up in a cruciform arrangement. Two conductors situated opposite one another form a pair, with respective electrical signals being transmitted on respective pairs. In other words the four conductors are arranged at the corners of a square in the cross-section of the star-quad, with the conductors making up a pair being arranged at diagonally opposed corners. The pairs of conductors thus lie perpendicular to one another and this produces a desired high damping of crosstalk from one pair to the other.
The star-quad cable is one of the symmetrical cables. In such cables, four conductors are twisted together in a cruciform arrangement. What this means is that the conductors situated in opposite positions form respective pairs of conductors. Because the pairs of conductors lie perpendicular to one another there is only a very low level of crosstalk. As well as the mechanical strengthening provided by the positioning of the conductors relative to one another, another advantage of the star-quad lay-up is its packing density, which is higher than with twisted pairs.
Because of the twist, the conductors, i.e. the individual cores, are longer than the cable itself. The so-called lay factor is the ratio of the length of an individual conductor to the length of the cable. In the case of telecommunications cables for example the lay factor is approximately 1.02 to 1.04. The lay factor correlates with a pitch or lead which is a result of the helical arrangement of the conductors which are twisted together. In the case of a thread, the pitch or lead specifies an axial distance between two thread grooves.
Known from printed document DE 14 90 692 A1, which is the generic document, is a carrier frequency cable having a central laid-up member in the form of a star-quad. Four stranded cores insulated with polyethylene are twisted with a lay length, s, and are surrounded by polyethylene insulation. The polyethylene insulation is enclosed by a shielding mesh of copper wires. A PVC sheath is applied to the shielding mesh.